docs: def _init_get_dict():
          """Initialize dictionary for filter operations."""
          get_dict = {'main chain': PandasPdb._get_mainchain,
                      'hydrogen': PandasPdb._get_hydrogen,
                      'c-alpha': PandasPdb._get_calpha,
                      'carbon': PandasPdb._get_carbon,
                      'heavy': PandasPdb._get_heavy}
          return get_dict